1. Field of the Invention
This invention relates to a toothed synchronous belt and pulley drive system and method of making such pulley particularly adapted for use with a toothed belt made primarily of polymeric material. It is particularly directed to small diameter pulleys, as hereinafter defined.
2. Prior Art Statement
Synchronous belt and pulley drives for mechanical power systems, utilizing toothed polymeric belts, are well known and widely used throughout industry. Typically, a system of this type utilizes a plurality of toothed pulleys and a cooperating toothed endless power transmission belt. The transmission of power is provided by the driving action between the pulley teeth and the belt teeth. Such systems are sometimes called positive drives, or positive indexing drives.
In a belt drive system of this type, each belt tooth sequentially engages the pulley teeth during normal operation which usually produces a line contact between each associated belt tooth and pulley tooth. In the usual application, the contact force is greater at or near the root of the belt tooth than near the crown, or crest, of such tooth. This greater contact force is often accompanied by relative sliding movement between the pulley tooth and the belt tooth at the root producing accelerated belt tooth wear and failure. Such wear and failure occurs routinely even in belt drive systems where pulley sizes and mounting space are not restricted in any manner.
However, many special drive systems have restrictive space, weight, and performance limitations which dictate that at least one pulley of the drive system be of the smallest possible size, i.e., smallest radius for the pulley teeth addendum circle. In using toothed belts with such small radius pulleys in these special systems, belt tooth wear and failure occurs many times more rapidly than in what may be considered a more normal system.
The most common practice in manufacturing a toothed pulley for use with a toothed belt has been to manufacture such pulley employing a nominal radius basically determined by the equation ##EQU1## In this equation P equals the pitch of the toothed pulley and belt at the pulley pitch circle, PLD equals the distance between the belt pitch line and the pulley teeth addendum circle on radius, and N equals the number of pulley teeth.
Once the radius has been thus determined, the major effort in prior pulley constructions has then been directed toward providing the belt teeth and/or pulley teeth with special shapes in an effort to provide improved operation. In addition, various attempts have been made in prior pulley constructions to provide special widening and deepening of pulley grooves, special undercutting of teeth, truncating of teeth, and the like, still for the purpose of providing improved operation. The following U.S. patents are examples of previous efforts:
(1) Patent No. 2,934,967 PA1 (2) Patent No. 2,937,538 PA1 (3) Patent No. 3,002,396 PA1 (4) Patent No. 3,117,460 PA1 (5) Patent No. 3,404,576 PA1 (6) Patent No. 3,744,330
Each of the pulleys described in items (1), (2), and (3) appear to be constructed with a nominal radius as described above and, efforts have been made to provide special shapes to either the pulley teeth, the belt teeth, or both. Item (4) appears to deal with the special shaping of belt teeth. Item (5) appears to teach pulleys with undercutting adjoining each pulley tooth. Finally, item (6) appears to teach widening and deepening of the pulley grooves.
However, it is known that an optimum pulley has certain desirable features which include construction thereof so that during operation with a cooperating belt the actual path of travel of a belt tooth is provided upon entry and exit free of belt motion changing forces caused by the pulley. In addition, such a optimum pulley should be constructed to provide a close belt tooth to pulley groove fit. Finally, such an optimum pull y should be constructed to provide belt flank to tooth flank contact yet without relative sliding motion therebetween.
As used in this application, the term "small diameter pulley" refers to a diameter less than the diameter of a pulley referred to in U.S. Pat. No. 2,934,967, Column 3, lines 50 to 56.